Design And Synthesis Of Six-membered N-heterocyclic Protic Ionic Liquids And Research On Ammonia Absorption Performance

Ammonia(NH3)is an important chemical raw material,but its direct emissions will also cause serious pollution.Direct emissions of NH3-containing exhaust gas in chemical production not only cause waste of resources,but also do harm to the environment.Based on the advantages of adjustable structure,low vapor pressure,good thermal stability and high gas solubility,etc.,ionic liquids(ILs)have received widespread attention in the field of gas separation and purification.Although a series of conventional ILs,hydroxyl ILs and metal ILs,etc.,functional ILs are used for NH3 absorption,there are problems of low absorption of conventional ILs,high viscosity of functional ILs,and complicated synthesis processes.The development of functional ILs with with high absorption,low viscosity and simple synthesis process is the current development trend.Protic ionic liquids(PILs)is a type of ILs formed by neutralization of Bronsted acid and Bronsted base,which not only has higher NH3 absorption,but also is easy to desorb,which provides a new method for NH3 absorption.Meanwhile,the current research on PILs mainly focuses on imdazolium-based PILs,etc.Therefore,designing efficient and recyclable novel PILs is the key to the research of NH3 separation by ILs.On the basis of the above background,In this thesis,novel protic ionic liquids with six-membered N-heterocyclic cations are designed and synthesized.The structure of the PILs are characterized.Besides,their NH3 absorption performance,the absorption mechanism,and reversibility criteria were studied.The results provide guidance for the design of new reversible ionic liquids.The main research contents and results of this thesis are as follows:(1)Design,synthesis and performance evaluation of six-membered N-heterocyclic PILs.Five kinds of PILs with six-membered N-heterocyclic cations were designed and synthesized,including pyridinium bis(trifluoromethylsulfonyl)imide([Py][NTf2]),2-methylpyridinium bis(trifluoromethylsulfonyl)imide([2-mPy][NTf2]),2,6-dimethylpyridinium bis(trifluoromethylsulfonyl)imide([2,6-2mPy][NTf2]),piperidinium bis(trifluoromethylsulfonyl)imide([Piper][NTf2]),2-(piperidiniumlyl)ethanol bis(trifluoromethylsulfonyl)imide([EtOHPi][NTf2]).The physicochemical properties of PILs were measured and their NH3 absorption performance were evaluated.During the NH3 absorption process,the two solid PILs will undergo a phase transition from solid to liquid,because the hydrogen bonding between NH3 and PILs destroys the own hydrogen bonding network of PILs.Under 40℃ and 0.10 MPa,the maximum NH3 capacity of pyridinium-based PILs is 3.078 mol NH3/mol IL,and the maximum NH3 capacity of piperidinium-based PILs is 2.406 mol NH3/mol IL;By comparing the NH3 capacities of pyridinium-based PILs,it was found that the substituents on the pyridine side chains had little effect on the NH3 absorption.Since the protic H on the pyridine ring are more acidic,the NH3 capacities of pyridinium-based PILs are higher than that of piperidinium-based PILs.However,Pyridinium-based PILs tend to decrease in NH3 capacity after saturation and in cycling tests,while piperidinium-based PILs can remain stable.(2)Studies on the NH3 absorption mechanism of PILs with six-membered N-heterocyclic cations.Through FR-IR,NMR and quantitative calculations,the mechanism of NH3 absorption by PILs was investigated.According to the change of FT-IR spectrum before and after absorption,it shows that PILs and NH3 form hydrogen bond.Further,the shift changes in 1H-NMR and DFT calculation results prove that protic H plays a major role in the absorption of NH3,which can form strong hydrogen bonds with NH3.The presence of hydroxyl groups are beneficial to the absorption of NH3;The FT-IR spectrum of[2-mPy][NTf2]after desorption cannot completely coincide with that of[2-mPy][NTf2]before absorption,indicating that[2-mPy][NTf2]cannot be completely regenerated.(3)Reasons for the decrease of NH3 capacity in pyridinium-based PILs and reversible absorption criteria.Through analysis of ESI-MS,it was found that NH4+was generated in the[2-mPy][NTf2]-NH3 systems after absorption,and 2-methylpyridine was found by 1H-NMR characterization of NH3 entrained gas.By comparing the 1H-NMR results of 2-methylpyridine,[2-mPy][NTf2]-NH3 supersaturated systems and pure[2-mPy][NTf2],it was found that the decrease was due to the pKa of the organic base corresponding to the cation was less than that of NH3,so the protic H being stripped by NH3 to form NH4+ in a supersaturated alkaline environment.Based on the above results,a criteria for the reversible absorption of six-membered N-heterocyclic PILs is proposed in combination with pKa:when the pKa of the organic base corresponding to the cation is less than the pKa of NH3,protic H will be stripped by NH3;otherwise,stable and reversible absorption can be achieved.